In the case of writing with a mechanical pencil, it is generally often the case that the mechanical pencil is not used in a situation where a body cylinder is perpendicular to a writing side (page), but used in a situation where the body cylinder is somewhat inclined to the writing side. In the case where the body cylinder is thus inclined for writing, there arises a phenomenon that a drawn line becomes bold as compared with that in the beginning, since the writing lead may locally abrade (partially wear) as the writing proceeds. Further, not only the drawn line changes in boldness, but also there arises a phenomenon that the drawn line changes in thickness (drawn line becomes thin) as the writing proceeds, since a contact area of the writing lead changes with respect to the writing side.
In order to avoid the above-mentioned problem, when the writing is carried out with the body cylinder being rotated, then it is possible to avoid such a problem that, as described above, the drawn line becomes bold as it is drawn, since a sharper side of the writing lead is rotatably in contact with the page when writing. However, when you write down with the body cylinder being rotated, there arises a problem in that operation of re-holding the body cylinder is required while the writing proceeds, leading to considerable reduction in writing efficiency.
In that case, it is not impossible to write down by re-holding the body cylinder and rotating it in a stepwise manner, in the case where exterior of the body cylinder is formed to be cylindrical. However, in the case of the mechanical pencil whose exterior may not be cylindrical and which may be designed to have a projection in the middle or which is a side-knock-type mechanical pencil, it is difficult to write by re-holding the body cylinder to be rotated in a stepwise manner as described above.
Now, Patent Documents 1 and 2 disclose a mechanical pencil having a rotational drive mechanism in which retracting operation of the writing lead is carried out to rotate the above-mentioned writing lead itself. According to the mechanical pencil as disclosed in Patent Documents 1 and 2, vertical projections and vertical grooves are arranged alternately in a body cylinder, and a cam part which has slopes, each being across the vertical projection and groove, is formed into the shape of a ring. Further, a rotor having formed thereon projections at intervals in a circumferential direction is accommodated in the body cylinder.
In this arrangement, by retreating the writing lead greatly (retracting it greatly), the above-mentioned rotor is pushed upwards within the body cylinder, and the projection of the rotor passes over the vertical projection formed at the cam part in the body cylinder and falls into the next groove via the above-mentioned slope, to thereby rotate the above-mentioned rotor. That is to say, in conjunction with the rotation of the above-mentioned rotor, the writing lead is rotationally driven.
However, according to the above-mentioned mechanical pencil, when the rotor is rotated, there is a problem in that the writing lead needs to have a large enough retreat stroke to allow the projection on the rotor side to pass over the vertical projection formed in the body cylinder. Thus, it is necessary to carry out the particular operation of rotating the writing lead when writing, and it is difficult to improve writing efficiency.
Then, the present applicant has proposed a mechanical pencil in which the writing lead is rotationally driven in one direction by of slight retreat and advance action of the writing lead caused by writing pressure and the writing efficiency is not affected; this is disclosed in Patent Document 3, for example.